I want to choose a good seat on a 8h+ flight in an Airbus A330-300. I use Seatguru to find it.
My preference (if I check in fast enough) would be the next row behind the lavatories row, as there's leg space aplenty and I might be able to sleep through most of the hours.
How bad is it to sit next to the lavatories on the long-haul, considering smell, noise and queues?
Mark's answer is kinda perfect, but I will give some extra information as a cabin crew member:
If you plan to sleep during the long haul: Take a window seat, no one will bother you to get out or in and you can rest your head on the side.
If you plan to stay awake to watch a movie or read a book: Take an aisle seat away from toilets.
If you plan to be awake for other reasons: Get a seat next to lavatories.
Firstly, be aware that there are three possible lavatory placings - at the front, midway in the plane, and at the back. The lavatory row at the back is 'special' as it's something people don't always realise - this row usually doesn't have reclining seats. Well worth noting. I regularly choose the back of the plane to get away from the kid-aisle with the bassinets further forward, but each flight has its own areas with their own merits. As a result of where I tend to choose, I am often near a bathroom.
However, it sounds like you're asking more about the mid-way lavatories, so I'll just describe lavatory seats in general.
Personally, I have anosmia (no sense of smell), so that side of things isn't a problem. Obviously the stronger your sense of smell, the worse it's going to get if there are issues with the bathroom.
Next, there's the inevitable queue. Although most planes have those handy signs to tell you when the toilet is in use, people still like to get up and queue - either because it's urgent for them, or even just to stand for a while and stretch those legs. What this means as a person close to the bathroom is that you constantly have people standing next to you, bumping you accidentally, watching your tv, chatting to each other right next to you, or potentially (horror!) even trying to make smalltalk with you! If you're trying to sleep, this can be very frustrating.
Nwxt there's the noise from the bathroom itself. You'll rarely hear actual ... 'functions' inside, but you will hear the flush, and some people have a tendency to slam the door open and make a racket.
It's also worth noting, as I alluded to before earlier, that midway bathrooms can sometimes be near the aisle with the bassinets and babies. This can be noisy and distracting as well.
So, long story short - if, like me, you don't even try to sleep on planes and have no sense of smell, I prefer the back of the plane (but not the very back row) and the aisle seat so that I can get up and wander a bit. If you like to sleep on a plane, don't watch movies, and have a stronger sense of smell, you may find it a very problematic and frustrating flight.
During a flight from New York to London I was assigned to seat 27B in an American Airlines B777 - the row directly in front of the lavatories. (http://www.seatguru.com/airlines/American_Airlines/American_Airlines_Boeing_777-200_A.php).
There was hardly any inconvenience from smell or from queues (at least nothing that I noticed). However, the noise was pretty annoying. It wasn't very loud but every two minutes or so you would hear the "swoooosh" sound from behind.
All in all not the best flying experience I ever had but also not totally unbearable.
I'll just add a few additional comments to complement the excellent answers:
The first row of a "group" of rows (right after an exit, lavatories, galleys, etc.) is often quite different from the others:
Regarding lavatories, one of the often decried aspects of seats next to them is people queuing next to them. This used to be a big issue (and still is on certain airlines/planes) with everybody getting up at the same time at the end of the movie(s). Nowadays, with personal screens (especially on long haul flights), people tend to spread out their use of the facilities a bit more as they not all "in sync". There should still be a bit of a queue at the end of the meal service, though.
Seats located next behind lavatories and seats located in front of lavatories are entirely different stories.
Behind The seats located behind lavatories are typically behind exit rows: extra leg room, screen (if applicable) in armrest and tray located in armrest. However, the area may be cold, does not have storage below seats, and can be a busy area. On modern planes, odors are typically not strong.
In front The seats in front of lavatories have almost no benefits, unless you are a frequent visitor to the facility. They don't recline, are busy, get knocked into, etc.
The question related to an A330 -- a very comfortable plane indeed, like most of the Airbus range.
There should be no problem being near the toilets -- and if you're referring to seats behind the toilets, then it should be even easier to spot a point at which there'll be no queue and use that slot for a visit to the toilet.
